The present invention relates to a radiation CT (computed tomography) scanner using radiations such as X-rays, and in particular, to a scanner in which spatial resolution can be adjusted in accordance with varying positions along the direction of the body axis of a patient.
An X-ray CT scanner has been widely used to obtain CT images in the medical field. The X-ray CT scanner generally has a gantry in which an X-ray tube and an X-ray detector are arranged for collecting X-ray projected data through a patient placed therebetween. The patient is laid on the tabletop of a couch, the tabletop being movable into an opening in the gantry. Also provided is a tabletop driving mechanism to move the tabletop at a given feeding speed. During examination, as the tabletop moves, the X-ray tube and X-ray detector are rotated around the patient on the basis of a given scan method. The X-ray projected data are sent to an image reconstruction unit, with the result that CT images can be reconstructed and displayed.
In one type of scanning method, a helical scan has been favorably employed. With the helical scan, while the X-ray tube and X-ray detector are rotated, the tabletop is moved along its longitudinal direction toward the opening of the gantry at a feeding speed in accordance with the slice thickness. That is, the X-ray tube and the X-ray detector are helically rotated around the patient.
In the helical scan of an X-ray CT scanner, X-ray projection data are continuously collected over a predetermined scan range in the direction of a patient's body axis (i.e., the longitudinal direction of the tabletop). After the data collection, CT images at any slice position can be reconstructed using the collected data.
In this conventional X-ray CT scanner, upper and lower slit members are disposed at positions close to the radiation output of the X-ray tube and to the input of the X-ray detector, so that the slits of both slit members form an X-ray fan beam having a predetermined slice thickness.
The pitch of the tabletop is selected beforehand depending on a desired slice thickness so as to accomplish continuous slices in the scan range. The "pitch" of the tabletop is used here to mean a distance traveling in its longitudinal direction for each rotation of the X-ray tube and X-ray detector with respect to a patient's body axis. In other words, the pitch is equivalent to the moving speed of the tabletop, though the expression differs.
The quantities, such as slit width, pitch, and amount of X-rays, have an effect on the spatial resolution of the CT images, and hereinafter such quantities, will be referred to as "resolution adjustment parameter".
In the X-ray CT scanner, when a portion having a complex skeleton such as auditory ossicles or a lesion is examined with higher spatial resolution, it is required that the above-mentioned slit widths and pitch be lowered and the amount of X-rays be increased. In contrast, examination of a portion of a non-complex skeleton such as a vertex portion or a portion around a lesion permits lower spatial resolution, thus the slit widths and pitch are large while the X-ray is relatively small.
However, in conventionally-used X-ray CT scanners, the resolution adjustment parameters (the slit widths, pitch, and X-ray amount) are fixed during scanning. There is thus a problem if an entire head or a wide region containing a lesion is to be examined using a single helical scan.
More specifically, at least one of small slit widths, a small pitch and a large amount of X-rays must be used with the helical scan in order to produce high-resolution images of auditory ossicles or a lesion. However, the drawback is that the scan time becomes longer, which sometimes causes the X-ray tube to exceed its heat capacity rating. This reduces the durability of the scanner. Further, unnecessary X-rays would be exposed to the vertex portion or the surrounding area of the lesion.
When performing CT scanning a patient is usually required to stop breathing during the scan. Where a scan range is mixed with precisely-examined portions and screening-examination portions, resolution adjustment parameters will be in focus on the precisely-examined portions. This will lead to a much longer scan time, and a patient is required to stop breathing for an uncomfortably long time.
On the other hand, when at least one of large slit widths, a large pitch and a small amount of X-rays is selected, it elongates the scan range in the body axis direction of a patient. This results in reducing spatial resolution at the portion of auditory ossicles or a lesion, whose image quality will be poor.
In order to avoid such reciprocity, a number of helical scans could be repeated for each diagnostic portion using different resolution adjustment parameters. But such repetition would cause the entire examination time to be much longer.